Deck mounted pull riser tensioning system

ABSTRACT

A deck mounted pull riser tensioning system for a floating platform or vessel includes at least two cylinders having a cylinder body supported at an upper or lower end on a deck or tensioner support structure of the platform or vessel in spaced apart relation with the cylinder rods extending downwardly from the cylinder body, or vice versa. A load structure member is connected to lower ends of the cylinder rods or bodies and has an opening therethrough for receiving a portion of a riser extending from the seafloor. A vertically adjustable riser attachment supported on the load structure member over the opening attaches to a portion of the riser extending through the opening and generally centrally between the cylinders. The cylinders pull upwardly on the load structure member to exert an upward tensioning force on the riser at the location of the riser attachment.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority of U.S. Provisional Patent Application Ser. No. 60/835,655, filed Aug. 3, 2006.

BACKGROUND OF THE INVENTION

1. Field Of The Invention

This invention relates generally to riser tensioner systems for offshore floating platforms and vessels, and more particularly to a deck mounted pull riser tensioning system for use on floating platforms and vessels that incorporates direct acting cylinders having cylinder rods connected with a load structure member that has a riser attachment which attaches to a riser extending therethrough and generally centrally between the cylinders whereby the cylinders exert an upward tensioning force on the riser.

2. Background Art

Marine riser systems are employed to provide a conduit from a floating platform or vessel at the water surface to the blowout preventer stack or, production tree, which is connected to the wellhead at the sea floor. A tensioning system is utilized to maintain a variable tension to the riser string alleviating the potential for compression and buckling or failure. Historically, conventional riser tensioner systems have consisted of cylinder assemblies with a fixed cable sheave at one end of the cylinder and a movable cable sheave attached to the rod end of the cylinder. The assembly is mounted on the vessel to allow routing of wire rope which is connected to a point at the fixed end and strung over the movable sheaves and routed via additional sheaves and connected at the terminal end to the riser via a support ring. Activation of the cylinder forces the rod and end sheave to stroke out thereby tensioning the wire rope and the riser.

Typically these types of tensioner units require frequent maintenance due to the constant motion producing wear and degradation of the wire rope members and occupy a relatively large space on the supporting structure. Available space for installation and, the structure necessary to support the units including weight and loads imposed thereby, particularly in deep water applications where the tension necessary requires additional tensioners.

There are several patents directed toward other types of riser tensioning systems that utilize cylinder assemblies connected with the riser that do not utilize the cable and sheave arrangement.

Widiner et al, U.S. Pat. No. 4,379,657 discloses a modular riser tensioner having a frame for mounting on a platform deck with air and oil accumulators mounted on a mounting frame and connected to cylinders also on the mounting frame. A piston located within the cylinders carries a riser tensioning ring for supporting the riser. Pairs of cylinders are located on opposite sides of the riser and connected to an oil accumulator independent of the other cylinders.

Myers et al. U.S. Pat. No. 4,883,387 discloses a tensioner system utilizing at least three tensioners each pivotally secured to both a lower surface of the production platform and to a tensioner ring that is itself secured to the riser. The tensioner ring may be generally octagonal with arms protruding from alternate faces of the octagon that define connecting points for the tensioners. The tensioners are angulated with respect to the axis of the riser, converging toward a single point lying on that axis and defining a first angle. The arms preferably form a second angle with respect to the body of the tensioner ring that is equal to the first angle so that the reaction surface defined by the bottom of the arms is perpendicular to the force lines along which the tensioners act.

Parikh, U.S. Pat. No. 5,310,007 discloses a tensioning ring and riser assembly for use in oil well platform riser tensioning apparatus. The riser has male threads formed on the exterior surface and the annular tensioning ring has female threads formed on the interior. The tensioning ring is tensioned in a first direction relative to the riser whereby hoop stress is uniformly distributed and minimized in both riser and tensioning ring with the shear load between them distributed over the wall thickness of both parts.

Pallini, Jr. et al, U.S. Pat. No. 5,551,803 discloses an apparatus for tensioning a riser that extends from a floating platform to a subsea wellhead has hydraulic cylinders. The hydraulic cylinders attach between the platform and the riser in a cluster with the axis of the riser substantially parallel with the axes of the hydraulic cylinders. A guide sleeve within the cluster transfers bending moments of the riser to the platform.

Thory, U.S. Pat. No. 5,846,028 discloses a controlled-pressure multi-cylinder riser tensioner having a plurality of preferably six control-cylinder units with proximal ends attached pivotally to a bottom surface of an operational floor and distal ends pivotally attached to a riser-tensioner ring. Pressure lines in communication with opposite ends of the control cylinders lead to sources of pressure that are separately controlled. Stroke length of the control-cylinder units is typically 50 feet. The control-cylinder units project downwardly into a moon pool to avoid obstruction of workspace on an operational floor of the vessel.

Otten et al, U.S. Pat. No. 6,045,296 discloses a tensioning device for applying tension to a riser from a surface deck of an offshore oil or gas well. The tensioning device is used with a riser of the type having threads formed on an exterior surface thereof. A tension ring of the tensioning device is formed from first and second ring halves that are clamped together about the riser and have inner threads that engage the threads of the riser. Tensioning members that couple at one end to the tension ring and at the other end to the surface deck are used to apply tension to the riser from the surface deck.

Reynolds, U.S. Pat. No. 6,530,430 discloses a tensioner/slip-joint module which includes at least one mandrel having at least one hang-off donut, at least one upper flexjoint swivel assembly, at least one radially ported manifold, at least one tensioning cylinder, and at least one slip-joint assembly combined in a single unit. The module compensates for vessel motion induced by wave action and heave and maintains a variable tension to the riser string.

Wetch et al, U.S. Pat. No. 6,688,814 discloses a substantially rigid riser to offshore platform connector and a process of using the connector that allows a riser to be preloaded and adjustably positioned relative to the offshore platform. The connector allows the riser tensile load to be limited by preloading and adjustably positioning the connector. In the preferred embodiment, hydraulic cylinders are used to preload the connector as well as to partially or fully support the riser during maintenance or repair procedures.

SUMMARY OF THE INVENTION

The present invention overcomes the afore mentioned problems and is distinguished over the prior art in general, and these patents in particular by a deck mounted pull riser tensioning system for a floating platform or vessel which includes at least two cylinders having a cylinder body supported at an upper or lower end on a deck of the platform or vessel in spaced apart relation with the cylinder rods extending downwardly from the cylinder body. A load structure member is connected to lower ends of the cylinder rods and has an opening therethrough for receiving a portion of a riser extending from the seafloor. A vertically adjustable riser attachment supported on the load structure member over the opening attaches to a portion of the riser extending through the opening and generally centrally between the cylinders. The cylinders pull upwardly on the load structure member to exert an upward tensioning force on the riser at the location of the riser attachment.

Alternatively, the tensioner cylinders may be inverted wherein the body of the cylinder is oriented downward and the rods extending upward and supported at an upper end on the deck or tensioner support structure of the platform or vessel in a spaced apart relation and the cylinder body disposed at the lower end of the rod and connected to the load structure member. The tensioner cylinders may be supported on the deck of the platform or vessel or on a tensioner support structure, which may be the deck itself or an appendage to the deck structure.

Several preferred embodiments are disclosed wherein the tensioner cylinders are disposed in a vertical, a near vertical, or a vertically offset orientation, and supported at a lower deck level or upper deck level of the platform or vessel and may be rigidly fixed to the supporting deck or supported by various bearing supports to substantially prevent or to allow relative rotational movement between the cylinders and the deck on which they are supported.

One of the advantages of the present system is that the tensioner cylinder arrangement uses multiple, direct acting cylinders configured in a manner to minimize the overall footprint of the system and to achieve maximum tensioner stroke with minimum overall length of the cylinders.

Another advantage of the present system is that the tensioner cylinder configuration incorporates an accumulator system to achieve a desired stiffness variation with cylinder stroke, and in applications where multiple riser tensioning systems are deployed on the platform or vessel, the accumulator system may be shared between tensioner cylinders.

Another feature and advantage of the present system is that the rods of the tensioner cylinders are connected to a common load structure member that in turn supports a riser attachment which is attached to the riser itself and allows the attachment to the riser be variably adjusted vertically thereon to accommodate variations in the riser stack-up relative to the seafloor.

Another feature and advantage of the present system is that the interface between the riser and the load structure member may be rigidly fixed, or supported by a spherical bearing connection to offer a degree of relative rotational freedom.

Another feature and advantage of the present system is that the riser portion above the riser attachment may be centralized by roller centralizers at or near the deck support level, and at other locations, and the top of the riser may be centralized by roller guides that react to the cylinders themselves, or if the riser is used for production of hydrocarbons, the roller guides may be attached to a production tree or blowout preventer at the top of the riser and movably engaged with the cylinders for providing lateral guidance and centralization of the production tree and/or blowout preventer assembly relative to the cylinders.

Other features and advantages of the invention will become apparent from time to time throughout the specification and claims as hereinafter related.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation view showing, somewhat schematically, the deck mounted riser tensioning system apparatus in accordance with a preferred embodiment of the present invention wherein the tensioner cylinders are vertical and are rigidly mounted to a lower deck of the vessel.

FIG. 2 is a perspective view showing, somewhat schematically, a floating facility with numerous risers with the present tensioner systems positioned vertically to maintain clearance for freedom of motion of the drilling rig on the upper deck of the vessel.

FIG. 3 is a perspective view showing, somewhat schematically, a typical tensioner arrangement with the placement of the tensioner cylinders close to the riser so that internal bending loads on the load structure may be minimized, allowing optimization of its weight.

FIG. 4 is a more detailed perspective view of the interface between the tensioner cylinders, the support deck, and the load structure, with an additional centralizer structure to provide further stability to the upper portion of the riser between the load structure and the guide near the top of the riser.

FIG. 5 is a side elevation view showing, somewhat schematically, the major components of a preferred embodiment of the riser tensioning system wherein the tensioner cylinders are supported above the deck, and illustrating how the guide rail structure interfaces with the tensioner cylinders.

FIG. 6A is a side elevation view showing, somewhat schematically, an alternate support for the tensioner cylinder that supports the cylinder body at a location intermediate its ends.

FIG. 6B is a side elevation view showing, somewhat schematically, an alternate bearing support for the tensioner cylinder that minimizes alignment tolerance requirements for the tensioner cylinder array and the load structure.

FIG. 7 is a side elevation view showing, somewhat schematically, the major components of a preferred embodiment of the riser tensioning system wherein the tensioner cylinders are disposed above the deck offset from a vertical orientation on gimbaled supports on the deck.

FIG. 8 is a side elevation view showing, somewhat schematically, the major components of a preferred embodiment of the riser tensioning system wherein the tensioner cylinders are supported by an upper deck of the platform or vessel.

FIG. 9 is a side elevation view showing, somewhat schematically, a modification of the riser tensioning system wherein the riser is supported by a gimbal support on the load structure.

FIG. 10A is a schematic side elevation showing an accumulator attached individually to a tensioner cylinder.

FIG. 10B is a schematic perspective view showing a bank of accumulators shared with multiple tensioner cylinders.

FIGS. 11A, 11B, and 11C are schematic top plan views illustrating examples of geometrical arrangements of the tensioner cylinders about the riser utilizing 2, 4 and 6 tensioner cylinders, respectively, whereby the arrangement of the cylinders about the riser generally remains symmetrical, but does not require equal spacing to provide mechanical load stability to the riser load structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Several preferred embodiments of the present deck mounted riser tensioning system are described herein for use in tensioning a riser connected with a floating oil and gas drilling and/or production platform or vessel. As described hereinafter, the riser tensioning system utilizes multiple (at least two), direct acting tensioner cylinders configured in a manner to minimize the overall footprint of the tensioner system and to achieve maximum tensioner stroke with minimum overall length of the cylinders. Several preferred embodiments are described below wherein the tensioner cylinders are disposed in a vertical, a near vertical, or a vertically offset orientation, and supported at a lower deck level or upper deck level of the platform or vessel and may be rigidly fixed to the supporting deck or supported by various bearing supports to substantially prevent or to allow relative rotational movement between the cylinders and the deck on which they are supported.

In the following discussion, the tensioner cylinders are shown and described, for purposes of example only, as oriented with the body of the cylinder oriented upward and the rods extending downward; however, it should be understood that the cylinders may be inverted wherein the body of the cylinder oriented downward and the rods extending upward.

Also, for purposes of example only, the tensioner cylinders are shown as being supported on the deck of the platform or vessel; however, it should be understood that the tensioner cylinders may be supported on the deck or on a tensioner support structure, which may be the deck itself or an appendage to the deck structure.

Referring to FIGS. 1 through 5 of the drawings by numerals of reference, there is shown somewhat schematically a first embodiment of the deck mounted riser tensioning system 10 in accordance with the present invention. In this embodiment, the tensioner cylinders 11 are disposed vertically and the cylinder bodies 11A are rigidly mounted at their lower ends to a lower deck D of the platform or vessel deck by bearing supports 12 or on a tensioner support structure, which may be the deck itself or an appendage to the deck structure. The number of tensioner cylinders 11 in the system is at least two, but may be three, four, five, six or more depending upon the desired redundancy and reliability. The cylinder bodies 11A themselves may extend up to near the underside of the drilling deck, and the cylinder rods 11B are longer than the cylinder bodies so as to extend through the deck D or tensioner support structure, and are connected at their lower ends to a common load structure member 13. In the illustrated example, the rod ends of the tensioner are shown attaching to the load structure member 13 with pins. These pins may be specially configured with multiple rotating surfaces to facilitate ease of alignment with their attachment points on the load structure member. Due to the potential environmental exposure to the extended cylinder rods, specialized rod coatings, or protective covers may be applied to the rods.

A vertically adjustable riser attachment assembly 20 is supported over an opening in the load structure member 13 and is attached to a portion of a riser R that extends upwardly from the seafloor and through the opening and the riser attachment assembly and generally centrally between the tensioner cylinders. In operation, the tensioner cylinders 11 pull up on the load structure member 13, which exerts an upward tensioning force in the riser R at the location of the riser attachment assembly 20.

The riser R extends upward from the riser attachment 20 on the load structure member 13, through a centralizer 30, such as a roller centralizer, on the deck D or tensioner support structure and to a point sufficiently high to allow a production tree and/or or blowout preventer (BOP) 40 at its upper end to clear the deck D when the riser tensioner cylinders are fully stroked out. The centralizer structure 30 on the cylinder support deck D or tensioner support structure extends over an opening large enough to allow passage of the riser attachment assembly 20.

The vertically adjustable riser attachment assembly 20 allows the attachment point of the riser R to be variably adjusted, to accommodate variations in riser stack-up relative to the seafloor. For example, as seen in FIG. 5, an upper portion of the riser R is externally threaded and the riser attachment assembly 20 includes an internally threaded locking flange or clamp device 20A threadedly engaged thereon which may be positioned in a vertical range over which the riser attachment assembly may be locked to allow for vertical space-out adjustment of the riser and load adjustment.

The riser attachment assembly 20 transfers the load from the load structure member 13 to the riser itself. Several types of clamp devices and length adjustment joint profiles may be used. Other examples include (but not limited thereto): a single-piece clamp consisting of a helical threaded nut with interior helical threads that interface with complementary threads on the outer surface of the tension and length adjustment joint; a “split nut” system where the clamp consists of a series of nut segments housed in a bowl, the nut segments have either helical threads or non-helical threads (circumferential grooves) on the inside surface that interface with complementary threads on the outer surface of the tension and length adjustment joint; a slip system clamp which consists of several slip segments that are housed inside a slip bowl, and the inside surface of the slip segments interface with a smooth outer surface of the tension and length adjustment joint; and a split clamp that has an interior profile that interfaces with a complementary profile on the outer surface of the tension and length adjustment joint.

In this embodiment of the invention, additional lateral support to the riser R is provided by a keel guide 50 supported on a keel guide frame 51 at a point near the keel of the platform or vessel (FIGS. 1, 2). This lateral support assists in maintaining efficient cylinder seals by reduction in side load on the tensioning cylinders. FIG. 2 shows a floating facility with numerous risers R and a common keel frame 51 at the level of the keel of the platform or vessel for locating the keel guides for each riser.

As shown in FIGS. 3 and 4, the present invention allows placement of the tensioner cylinders 11 close to the riser R so that internal bending loads on the load structure member 13 may be minimized, allowing optimization of its weight. The minimum possible spacing of the tensioner cylinders may be controlled by the size of the Christmas tree or BOP 40 and required access around the tree or BOP, as well as the need to pull the tree or BOP out from between the tensioner cylinders for installation or replacement.

As best seen in FIG. 5, an upper centralizer and guide structure 60 having rollers 61 at its outer ends movably engaged with the tensioning cylinders may be connected to an upper end portion of the riser R to provide lateral guidance and centralization of the upper end portion of the riser relative to the cylinders 11. A guide rail 62 may be provided on the inner facing exterior side of the tensioner cylinder bodies 11A in which the rollers are received, and may extend above the maximum height of the cylinders themselves. If the upper end portion of the riser R includes a Christmas tree and/or a blowout preventer (BOP) assembly 40, the upper centralizer and guide 60 may be in the form of a work platform at the base of the Christmas tree or BOP 40, to allow access to the tree body for maintenance, etc., with rollers 61 mounted at the outer ends of the work platform movably received in the guide rails 62 on the cylinders. Alternatively, the upper centralizer and guide 60 may be in the form of arms attached to the Christmas tree or BOP 40 having rollers 61 at their outer ends movably received in the guide rails 62 on the cylinders. The upper centralizer and guide structure 60 may be adjustable to allow for vertical clear passage of portions of the riser of different diameters.

It should be understood that the riser tensioning system described herein is configured in such a fashion that the entire riser R may be passed vertically through the tensioner cylinder assembly array as the riser is assembled in the drill rig on the vessel. This is achieved by providing the centralizer structures described above to have sufficient adjustment capacity to fit over the largest component of the riser. It should also be noted that the riser attachment assembly 20 on the load structure member 13 is also of sufficient size to allow passage of all riser components.

Alternatively, large riser components (typically found near the lower end of the riser) may be pre-assembled, then lowered and raised through the keel guide 50 and load structure member 13. This would allow minimization of the required opening of the centralizer 30 and riser attachment assembly 20. The riser attachment assembly 20 described above may be used as a means of temporary support for a partially assembled riser string as it is deployed, prior to landing and attachment of the riser to a point on the sea floor.

The tensioner cylinder mounting arrangement allows simple servicing of the cylinder rod seals as well as replacement of an individual cylinder. The cylinder seals are accessible for service from beneath the support deck or tensioner support structure, which may be reached by a temporarily placed work platform. Cylinder replacement is possible by release of an individual cylinder from the load structure, and vertical lift of the entire cylinder.

In the illustrated examples discussed above, the tensioner cylinders 11 are located vertically to allow clearance as required with other equipment and operations on the vessel. A common clearance requirement is to permit lateral motion of a drill rig (on the uppermost level of the platform or vessel topsides) over the tensioner cylinders clustered around each riser. Thus, to meet this requirement, the tensioner cylinders may be supported at a lower deck level, or at a subdeck level. If necessary, the cylinders may also be supported by an additional framework at another appropriate level.

Tensioner cylinder length is determined by stroke requirements that result from the motion characteristics of the specific floating platform or vessel. The tensioner cylinders are preferably placed as high as possible in order that interaction between waves and the tensioning system is minimized.

FIG. 6A shows a modification of the location of the bearing support for the tensioner cylinders 11. In this modification, an annular flange 11C is disposed on the exterior of the cylinder body 11A at some point intermediate its ends and is supported on the deck D or tensioner support structure. This modified bearing support may be utilized in installations with more limited vertical space between the tensioner support deck or structure and the drill deck above.

FIG. 6B shows another alternate bearing support for the tensioner cylinders 11. In this modification, the bearing support may be either a spherical flange 11D and seat 12A bearing or gimbaled support, as shown, or other conventional types of flexible support bearings commonly available. This modification provides the tensioner cylinders with a certain amount of rotational motion with respect to a pure vertical configuration. This type of gimbaled bearing support is useful for minimizing alignment tolerance requirements for the tensioner cylinder array and the load structure member. Furthermore, this type of cylinder support on the deck or tensioner support structure may be used if a keel guide structure is not desired. Omission of the keel guide would allow alignment of the tensioner cylinder assembly with the riser during lateral offset of the vessel.

FIG. 7 shows another alternate bearing support for the tensioner cylinders 11. In this modification, the tensioner cylinders 11 are offset from vertical, and supported by gimbaled supports 11D, 12A, as described above, with their rod ends converging downward and attached to the load structure member. This modification allows further reduction in size of the load structure member. In addition, the position of the opposing tensioner cylinders provides lateral stability to the riser relative to the support deck. In this embodiment of the invention, the guide rail structure on the cylinders is not used.

FIG. 8 shows another alternate embodiment of the support for the tensioner cylinders, wherein the upper ends of the tensioner cylinders 11 are supported by a chain or other flexible member beneath an upper deck or tensioner support structure of the platform or vessel. In this embodiment of the invention, the entire tensioner cylinder assembly is free to move laterally as the vessel offsets. The components previously described are assigned the same numerals of reference, but will not be described again in detail to avoid repetition.

FIG. 9 shows a modification of the vertically adjustable riser attachment assembly 20 wherein the upper portion of the riser R is externally threaded and the riser attachment assembly 20 includes an internally threaded gimbaled support flange 20B supported on a spherical seat 20C on the load structure member 13 to allow a certain amount of rotational motion of the riser with respect to a pure vertical configuration. This gimbaled riser support modification provides reduction of bending stresses in the riser without the need for the keel guide structure.

The present riser tensioning system also incorporates an accumulator system as necessary to achieve a desired stiffness variation with cylinder stroke. The accumulators have not been shown in the previously described drawing figures to avoid confusion and to more clearly illustrate the other components of the system. As seen in FIG. 10A, accumulators 70 may be attached to each individual tensioner cylinder 11, or, in applications where multiple tensioner systems are deployed on the vessel, the accumulators 17 may be provided as a bank of accumulators shared between several tensioner cylinder assemblies (FIG. 10B).

As discussed above, the riser tensioning system requires a minimum of two opposing tensioner cylinders to provide mechanical load stability to the riser load structure member 13. As the number of load cylinders is increased, the geometrical arrangement of the cylinders about the riser R generally remains symmetrical, but does not require equal spacing. FIGS. 1 1A, 11B, and 11C are top plan views illustrating schematically examples of geometrical arrangements of the tensioner cylinders 11 about the riser R utilizing 2, 4 and 6 tensioner cylinders, respectively, whereby the arrangement of the cylinders about the riser generally remains symmetrical, but does not require equal spacing to provide mechanical load stability to the riser load structure member 13. Placement of the cylinders in non-equal spacing allows a minimum overall footprint of the tensioner system, while allowing an increased opening for insertion and removal of the larger diameter riser components.

It should be noted that removal and replacement of tensioning cylinders may be facilitated by removal of multiple opposing cylinder pairs in an array of cylinders to allow equalization of load on the remaining cylinders without imposing bending stresses on the riser itself. Alternatively, a single cylinder might be removed from the array, and pressures offset in the remaining cylinders to minimize bending stresses in the riser.

In the preceding discussion, the tensioner cylinders have been shown and described, for purposes of example, as oriented with the body of the cylinder oriented upward and the rods extending downward; however, it should be understood that the cylinders may be inverted wherein the body of the cylinder oriented downward and the rods extending upward.

For example, the riser tensioning system may have at least two hydraulic or pneumatic cylinders with the cylinder rods supported at an upper end in a generally vertical orientation beneath a the deck or tensioner support structure of the platform or vessel in a spaced apart relation and the cylinder body disposed at the lower end of the rod. In this modification, the load structure member 13 is connected to the lower ends of the cylinder bodies.

Also, for purposes of example only, the tensioner cylinders have been shown as being supported on the deck of the platform or vessel; however, it should be understood that the tensioner cylinders may be supported on the deck or on a tensioner support structure, which may be the deck itself or an appendage to the deck structure.

While this invention has been described fully and completely with special emphasis upon preferred embodiments, it should be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described herein. 

1. A deck mounted pull riser tensioning system for a floating platform or vessel, comprising: at least two hydraulic or pneumatic cylinders having a cylinder body supported at a lower end on a tensioner support structure of said platform or vessel in spaced apart relation and having cylinder rods extending downwardly from said cylinder body lower ends through the deck; a load structure member connected to lower ends of said cylinder rods and having an opening therethrough for receiving a portion of a riser extending from the seafloor; and a vertically adjustable riser attachment supported on said load structure member over said opening for attachment to a portion of a riser extending through said load structure opening and generally centrally between said cylinders, said riser attachment adjustably positioned on the riser to accommodate variations in riser stack-up relative to the seafloor; said cylinders, when activated, pulling upwardly on said load structure member to exert an upward force on the riser at the location of the riser attachment.
 2. The deck mounted pull riser tensioning system according to claim 1, further comprising: riser centralizer means disposed above said riser attachment for receiving and guiding an upper portion of the riser between said cylinders.
 3. The deck mounted pull riser tensioning system according to claim 1, wherein said cylinders are disposed substantially vertical with respect to the deck on which they are supported.
 4. The deck mounted pull riser tensioning system according to claim 1, further comprising: riser guide means disposed below said load structure member for receiving and providing lateral guidance and centralization of the riser below said load structure member.
 5. The deck mounted pull riser tensioning system according to claim 1, further comprising: upper guide means adapted for connection to an upper end portion of the riser and having outer ends movably engaged with said cylinders for providing lateral guidance and centralization of the upper end portion of the riser relative to said cylinders.
 6. The deck mounted pull riser tensioning system according to claim 5, wherein the upper end portion of the riser includes a Christmas tree and/or a blowout preventer (BOP) assembly; and said upper guide means is adapted for connection to the Christmas tree and/or blowout preventer (BOP) assembly has outer ends movably engaged with said cylinders for providing lateral guidance and centralization of the Christmas tree and/or blowout preventer (BOP) assembly relative to said cylinders.
 7. The deck mounted pull riser tensioning system according to claim 1, wherein said riser attachment is supported on said load structure by a gimbal support.
 8. The deck mounted pull riser tensioning system according to claim 1, wherein said cylinders are supported on said deck so as to substantially prevent relative rotational movement between said cylinder body lower ends and the deck on which they are supported.
 9. The deck mounted pull riser tensioning system according to claim 1, wherein said cylinders are supported on said deck so as to allow relative rotational movement between said cylinder body lower ends and the deck on which they are supported.
 10. A deck mounted pull riser tensioning system for a floating platform or vessel, comprising: at least two hydraulic or pneumatic cylinders having a cylinder body supported at an upper end in a generally vertical orientation beneath a tensioner support structure of said platform or vessel in a spaced apart relation and having cylinder rods extending downwardly from lower ends of said cylinder body; a load structure member connected to lower ends of said cylinder rods and having an opening therethrough for receiving a portion of a riser extending from the seafloor; and a vertically adjustable riser attachment supported on said load structure member over said opening for attachment to a portion of a riser extending through said load structure opening and generally centrally between said cylinders, said riser attachment adjustably positioned on the riser to accommodate variations in riser stack-up relative to the seafloor; said cylinders, when activated, pulling upwardly on said load structure member to exert an upward force on the riser at the location of the riser attachment.
 11. The deck mounted pull riser tensioning system according to claim 10, further comprising: riser centralizer means disposed above said riser attachment for receiving and guiding an upper portion of the riser between said cylinders.
 12. The deck mounted pull riser tensioning system according to claim 10, further comprising: riser guide means disposed below said load structure member for receiving and providing lateral guidance and centralization of the riser below said load structure member.
 13. The deck mounted pull riser tensioning system according to claim 10, further comprising: upper guide means adapted for connection to an upper end portion of the riser and having outer ends movably engaged with said cylinders for providing lateral guidance and centralization of the upper end portion of the riser relative to said cylinders.
 14. The deck mounted pull riser tensioning system according to claim 13, wherein the upper end portion of the riser includes a Christmas tree and/or a blowout preventer (BOP) assembly; and said upper guide means is adapted for connection to the Christmas tree and/or blowout preventer (BOP) assembly has outer ends movably engaged with said cylinders for providing lateral guidance and centralization of the Christmas tree and/or blowout preventer (BOP) assembly relative to said cylinders.
 15. The deck mounted pull riser tensioning system according to claim 10, wherein said riser attachment is supported on said load structure by a gimbal support.
 16. A deck mounted pull riser tensioning system for a floating platform or vessel, comprising: at least two hydraulic or pneumatic cylinders having a cylinder body supported on a tensioner support structure of said platform or vessel in spaced apart relation and having cylinder rods extending downwardly from said cylinder body lower ends through said tensioner support structure; a load structure member connected to lower ends of said cylinder rods and having an opening therethrough for receiving a portion of a riser extending from the seafloor; and a vertically adjustable riser attachment supported on said load structure member over said opening for attachment to a portion of a riser extending through said load structure opening and generally centrally between said cylinders, said riser attachment adjustably positioned on the riser to accommodate variations in riser stack-up relative to the seafloor; said cylinders, when activated, pulling upwardly on said load structure member to exert an upward force on the riser at the location of the riser attachment.
 17. A deck mounted pull riser tensioning system for a floating platform or vessel, comprising: at least two hydraulic or pneumatic cylinders having a cylinder rod supported at an upper end in a generally vertical orientation beneath a tensioner support structure of said platform or vessel in a spaced apart relation and having a cylinder body extending downwardly from said cylinder rod; a load structure member connected to lower ends of said cylinder body and having an opening therethrough for receiving a portion of a riser extending from the seafloor; and a vertically adjustable riser attachment supported on said load structure member over said opening for attachment to a portion of a riser extending through said load structure opening and generally centrally between said cylinders, said riser attachment adjustably positioned on the riser to accommodate variations in riser stack-up relative to the seafloor; said cylinders, when activated, pulling upwardly on said load structure member to exert an upward force on the riser at the location of the riser attachment. 